This invention relates to a fan for a smoke and vapour extraction system. It also relates to an extraction system provided with such a fan.
With particular reference to systems for the extraction of smoke and vapours which are generated during the preparation of food on a cooking surface (systems also indicated below as extraction hoods), these comprise a structure which can be combined below with a kitchen cabinet or which can be inserted into such a kitchen cabinet, or which may form part of the kitchen furniture itself or may be an independent structure.
Such a hood comprises a fan which extracts smoke and vapours from the cooking surface and directs them towards a discharge conduit which may open into the environment in which the surface is located or outside the said environment; in that case the discharge conduit is associated with a pipe that transfers the said smoke and vapours outside the said environment.
A normal fan for an extraction hood comprises a diffuser (or volute) containing an impeller defined by a finned body (or one with blades) associated with an electric motor and located in an opening in the diffuser through which the smoke or vapours are extracted. Close to such opening there is in the structure of the hood at least one grease-trap filter, at least one other filter which can be located in another suitable position of the diffuser when the said smoke or vapours are returned to the environment in which the cooking surface is located.
The impeller has an inlet cross-section for such smoke and vapours which may be to a greater or lesser extent close to such a filter. Although suitably and adequately performing their function in the extraction hoods available on the market, known fans have limitations and problems of various kinds.
One particularly felt problem is that of having a high volumetric throughput to quickly extract smoke from the cooking surface, having a high fan efficiency and consequently a low energy efficiency class.
The new guidelines introduced by the “energy label” for the energy efficiency of extractor units (EU Regulation 65/2014 and Ecodesign Regulation 66/2014) have resulted in companies working to increase the efficiency of existing fan systems. In the past improvements in performance were generally achieved through optimising the motors, in line with technical knowledge.
Because the overall efficiency of an extractor unit is defined as follows:ηg=η·ηm whereηg: overall efficiencyη: fluid dynamic efficiencyηm: motor efficiency,given that motors have very high efficiency values at the present time, the margins for any possible improvements in overall efficiency are somewhat low.
For this reason companies are currently investing to optimise the fluid dynamic efficiency (η) of extractor units. At the new mould making stage fluid dynamic optimisation of the diffuser and impeller initially gives rise to greater investment costs, but these are largely recovered thanks to the fact that the costs of a mould, even one of excessive geometrical complexity, are approximately the same, regardless of any optimisation of the shapes. Instead an improvement in the performance of a motor is in most cases associated with an increase in the fixed cost of each individual component.
On the basis of the above, the aims of fluid dynamic optimisation are those of:                i) for the same motor, achieving a rise to the next higher efficiency class; and        ii) for the same energy efficiency class, using a less powerful motor, which generally has a lower cost.        
For fluid dynamic efficiency to be improved, action needs to be taken on the fan and its components such as the impeller and diffuser. This however often involves the construction of very complex moulds (with a high number of undercuts) to produce the components of a high efficiency fan. Consequently the costs of such moulds are very high, and this has an effect on the finished product.
Another way of increasing the efficiency or fluid dynamic performance is also to produce impellers having the particular shape which frequently give rise to dissipative energy phenomena within the diffuser, in the form of turbulent vortices, which actually reduce performance in terms of volumetric throughput, static pressure and electrical energy consumption.
Another problem which is encountered when searching to optimise the procedure for manufacturing a fan for extraction hoods is associated with the mechanics of the fan's own structure. Fans often in fact have geometries which have an adverse effect on any overall efficiency of the extraction hood due for example to the excessively small distance between the fat-trap filters located at the inlet to the hood and the extraction cross-section of the fan (or the aperture of the latter corresponding to the impeller).
In addition to this, the structure of the hood and/or the fan may have an adverse effect on cooling of the impeller's electric motor (and therefore on its efficiency) or give rise to head losses due to the presence of an extraction grid in the inlet cross-section of the impeller (associated with the diffuser) and/or other details (such as motor support, cable duct, etc.).
Known fans therefore have other disadvantages associated with the complex assembly of their components (owing to the presence of various individual parts, such as for example the motor, impeller, diffuser supporting cage, which must be attached to the diffuser itself) and large size, giving rise to the hood in which the fan is inserted being of non-negligible size.
Furthermore, again with regard to the fluid dynamic efficiency of known fans, these have an impeller with the normal configuration of double parallel discs between which blades in an inclined arrangement with respect to the radii of such discs are located. The electric motor is placed between the latter. This impeller conformation, which can be achieved through complex and relatively long production and assembly means, which are therefore of more than negligible cost, results in the presence in the diffuser of at least one internal crown (which guides the flow of air to the outlet from the diffuser) having an adverse effect on the energy of the outgoing air flow and giving rise to dissipative energy phenomena in the form of turbulent vortices created between the blades of the impeller and such crown. This reduces the performance of the fan in terms of volumetric throughput and static pressure.
For the above-mentioned reasons there is also an increase in the noise level of the fan. Such noise is also due to the distribution of blades in the impeller, and in particular the distribution of the blades between the double discs: this distribution, often with the blades equally distant, gives rise to a tonal frequency or noise that is readily audible to human ears when the extraction system is in use, a noise which is also a nuisance and disturbs everyone located close to such a system.
US 2011/052385, U.S. Pat. No. 1,893,184 and US2015/260201 each describe a fan comprising an impeller located within a container body or diffuser driven by its own actuator; the impeller has blades arranged radially and spaced apart in a non-uniform manner. This makes it possible to reduce the noise from the fan when in use.
The abovementioned priorities also describe or illustrate a diffuser having an increasing diameter in a plane cutting through it at right angles to the axis of rotation of the impeller. However a cross-section in a plane containing that axis always has the same dimensions all along the diffuser and at least from the inlet to a delivery conduit from the diffuser to its outlet. This gives rise to the possibility of head losses, which have an effect on fluid dynamic efficiency and therefore an overall effect on the fan.
At least US2011/052385 and U.S. Pat. No. 1,893,184 show prior art that form the starting point for the improvements of the present invention.